Radio frequency identification (RFID) tags are found in an ever-increasing number of applications within a wide variety of different environments. Within the oilfield such tags can be used to identify equipment and/or transfer information along the well bore. The use of RFID tags in well bore flows poses a number of challenges not encountered in the field of inventory control and device tracking where such tags are commonly applied.
The flow of effluents from the well bore is often turbulent and the effluents may consist of multiple phases such as hydrocarbons, water and gas. RFID tags are convected along in the flow of the produced fluids. At a surface location the tags are energized and read with an appropriate reader antenna. The lack of orientation of a device free to move within the flow, the temperature of the flow, and the metal pipe surrounding the flow are all factors that either alone or jointly prevent the use of most of the known RFID tags. For example, known tags often have simple antennas with a directional characteristics. In conventional inventory control systems it is possible to overcome this problem by controlling the orientation. For most of these known systems it can be assumed that the tag does not change its orientation during the read-out phase, which is typically in the order of 0.05 second at the low frequencies of interest. However as a tag in a turbulent flow will tumble, it may not couple to the field produced by the reader antenna. In a turbulent flow it is likely that the tag changes its orientation during the read-out phase thus losing contact with the reader. While it is possible to shorten the read times by using higher frequencies, such frequencies have a higher attenuation in conductive fluids and are therefore not a practical solution for many oilfield applications.
A number of technologies have been proposed around the idea of sending some object or element up or down the borehole. A raw piece of semiconductor memory onto which data is written by a downhole device has been disclosed for example in GB Patent Application 1 549 307. A more sophisticated and robust vessel containing memory has been disclosed by GB Patent No 2, 352, 041 and U.S. Pat. No. 6,971,265 assigned to Schlumberger Technology Corporation. Alternatively, even more complex vessels containing a variety of sensors and data storage have been disclosed. Examples of this solution can be found in GB Patent 2 352 042, co-pending U.S. patent application. Ser. No. 10/030,587 assigned to Schlumberger Technology Corporation, and U.S. Pat. No. 6,241,028.
U.S. Pat. No. 6,443,228 discloses the use of flowable devices in well bores to provide communication between surface and downhole instruments, among downhole devices, to establish a communication network in the well bore, to act as a sensor, and to act as power transfer devices. In some embodiments, the upward communication is proposed by writing information on the flowable devices downhole which are bound for the surface.
GB Patent No. 2 396 170 (incorporated herein by reference) discloses a well control system enabling the control of various downhole well control functions by instructions from the surface. The well or downhole tool conveyance mechanism can be used without necessarily being equipped with electrical power and control cables extending from the surface and without the use of complex and inherently unreliable mechanical shifting or push/pull techniques requiring downhole movement controlled remotely from the surface. The invention of this co-pending application makes use of downhole well control apparatus that is responsive to instructions from elements such as fluids or physical objects such as darts and balls that are embedded with tags for identification and for transmission of data or instructions. According to at least one disclosed embodiment, a downhole device may also write information to the element for return to the surface. In these disclosed embodiments, where information is being sent from a downhole location to the surface, information is written to the device (or acquired by the device itself) downhole.
U.S. Pat. No. 6,915,848, under obligation to assign to Schlumberger Technology Corporation (incorporated herein by reference) discloses a well control system enabling the control of various downhole well control functions by instructions from the surface without necessitating the well or downhole tool conveyance mechanism being equipped with electrical power and control cables extending from the surface and without the use of complex and inherently unreliable mechanical shifting or push/pull techniques requiring downhole movement controlled remotely from the surface. The invention of this co-pending application makes use of downhole well control apparatus that is response to instructions from elements such as fluids or physical objects such as darts and balls that are embedded with RF tags for identification and for transmission of data or instructions. According to at least one disclosed embodiment, a downhole device may also write information to the element for return to the surface.
GB Patent No. 2 396 170 discloses a downhole communication system with a plurality of releasable vessels positioned at downhole locations, the vessels containing signal information affixed to the vessels prior to placement of the vessels downhole, and the signal information indicating the presence of at least one of three or more predetermined downhole conditions. A detecting system is positioned on the surface such that the signal information can be detected on one or more of the vessels. A processing system is located on the surface and is programmed to establish the presence of the predetermined downhole condition based on the signal information. The vessels can carry RF tags.
In the light of the above prior art, it is seen as an object of the present invention to provide RF based communication devices with improved read-out capabilities for use in turbulent flows. It is seen as a further object of the invention to provide tags that are more stable during a read-out phase.